Structural diversity and supramolecular aggregation in calcium, strontium, and barium salicylates incorporating 1,10-phenanthroline and 4,4'-bipyridine: probing the softer side of group 2 metal ions with pyridinic ligands

Abstract

Group 2 metal complexes [Ca(SA)₂(phen)]_n (1), [Sr₂(SA)₄(phen)₄] (2), and [Ba(SA)₂(phen)₂]_n (3) (SA = salicylate) have been obtained by the addition of 1,10-phenanthroline (phen) to the corresponding metal salicylates, while the bipyridine derivatives {[Ca₃(SA)₆(H₂O)₄](4bpy)₂}_n (4), {[Sr(SA)₂(H₂O)₃](4bpy)_1.5(H₂O)}_n (5), and {[Ba(SA)₂(H₂O)₃]-(4bpy)_1.5(H₂O)}_n (6) have been synthesized starting from the respective metal carbonates, salicylic acid (SA-H), and 4,4'-bipyridine (4bpy). The new compounds have been characterized by elemental analysis, pH measurements, thermal analysis, and spectroscopic measurements (IR, NMR, ultraviolet, and fluorescence). Molecular structure determination by single-crystal X-ray diffraction has been carried out for all the compounds. The thermal analysis studies indicate the loss of coordinated and/or lattice water molecules below 200 °C in 4-6 and the absence of any coordinated or uncoordinated water molecules in compounds 1-3. Compounds 1 and 3-6 exist as one-dimensional polymers while compound 2 crystallizes as a discrete dimer. Considerable variations have been observed in the molecular structures of 1-6 in terms of the geometry around the metal, the binding mode of salicylate, and the coordination behavior of the pyridine ligand. Calcium ion is hexacoordinated in 1, while in 4 both hexa- and heptacoordinated calcium ions are simultaneously present. Strontium exhibits coordination numbers of nine and eight in 2 and 5, respectively. The barium ions in 3 and 6 assume coordination numbers of eight and seven, respectively. While the OH group of the salicylate ligand does not directly bind the metal in 1-3, it coordinates to the metal ions in complexes in 4-6 in the un-ionized form. The 4bpy molecules show no direct ligation to the metal in 4-6; the phen ligands in 1-3, however, occupy one side of the coordination sphere around the metal. The presence of additional O-H···O, C-H···O, and N-H···O hydrogen bonding and ∏-∏ stacking in these compounds results in the formation of polymeric structures. The results obtained for the calcium complexes in this study have been compared with the available data in structural calcium chemistry with the aid of a detailed analysis of the Cambridge Structural Database.